The present invention relates generally to an imaging optical system lending itself to imaging apparatus using imaging devices such as CCDs or CMOSs, and more particularly to an imaging apparatus equipped with such an imaging optical system.
Surveillance cameras, cameras mounted on interphones, and on-board cameras are now gaining popularity. For instance, the on-boards cameras are being used as visual identification cameras plus higher sensor cameras used for detecting white lines, and keeping watch over swerving from lanes, obstacles, and drivers.
Optical systems used with these cameras require a fisheye optical system having a half angle of view of, e.g., about 80° or greater for the purpose of making the visible angle of view wide thereby eliminating or reducing blind spots. There is also mounting demand for an imaging optical system that is not only compact but also works for cost reductions. Such imaging optical systems having a half angle of view of the order of 80° are known from Patent Publications 1, 2, 3 and 4.    Patent Publication 1: JP(A) 2002-244031    Patent Publication 2: JP(A) 2005-227426    Patent Publication 3: JP(A) 2006-259704    Patent Publication 4: JP(A) 2007-101920
However, the imaging optical system set forth in Patent Publication 1 has a long whole length relative to its focal length, and so goes against the reductions of general size, and especially diametrical size. Patent Publication 2 shows imaging optical systems, most of which comprise five singles lens and are hard to decrease in size. Patent Publication 2 also discloses an imaging optical system comprising four single lenses; however, it has a long whole length relative to its focal length, and so goes against the reductions of general size, and especially diametrical size. For the imaging optical system set forth in Patent Publication 3, lenses having large aspheric biases are used for correction of aberrations, and so fabrication costs are on the increase. The imaging optical system disclosed in Patent Publication 4 is susceptible of cost rises because of using two or more glass lenses.
Having been made with such problems in mind, the present invention has for its one object to provide an imaging optical system that works more in favor of size reductions and cost reductions while making sure a wide angle of view and optical performance. Another object of the invention is to provide an imaging apparatus incorporating such an imaging optical system.